Abstract

In this thesis we analyze a form of non magneto hydrodynamic turbulence which could be described as disk weather since it forms vortices due to baroclinic effects. We want to find out if and how these vortices influences planet formation. The focus is on angular momentum transport and how efficient vortices can trap particles.

We estimate disk properties from observations and derive radial Brunt-Väisälä frequencies as well as cooling time-scales. Then we analyze the baroclinic amplification of vortices and the particle concentration therein. We use 2D as well as 3D local shear- ing box simulations with the PENCIL CODE to investigate the problems.

In 2D, we conduct a comprehensive study of a broad range of various entropy gradients, thermal cooling and thermal relaxation times covering the parameter space relevant for protoplanetary disks. We measure the Reynolds stresses as a function of our control parameters and see that there is angular momentum transport even for entropy gradients as low as β ≡ −d ln S/d ln r = 1/2. The amplification-rate of the perturbations appears to be proportional to β^2. The saturation level of Reynolds stresses on the other hand seems to be proportional to β^1/2. All entropy gradients will lead to Reynolds stresses of 10^−3 − 10^−2 which shows that baroclinic vortices are a feasible mechanism for transporting angular momentum.

The concentration of particles of different sizes in baroclinic vortices is first analyzed in 2 dimensions. Because we expect strong particle accumulations, particle feedback onto the gas is included. Particles accumulate inside the vortices and the local dust-to-gas ratios become high enough to trigger the streaming instability even for initial dust-to-gas ratios as low as ε_0 = 10^−4.

In 3 dimensional unstratified gas simulations we verify previous result. Once particles, that feel vertical gravity, with normalized friction times of St = 0.05and St = 1.0, and ε_0 = 0.01 are included, the vortex column in the mid-plane is strongly perturbed. Yet, when the initial dust-to-gas ratio is decreased the vortices remain stable and function as efficient particle traps. Streaming instability is triggered even for the lowest ε_0 = 10^−4 and smallest particle sizes (St = 0.05) we assumed, showing a path for planetesimal formation in vortex cores from even low global amounts of cm-sized particles.